Cytokines in Immune Response: Classification, New Discoveries, and Measurement

Cytokines are the molecular language of the immune response — small secreted proteins, typically 5–70 kDa, that coordinate every phase of inflammation, immunity, and tissue repair across autocrine, paracrine, and endocrine signaling modes. From a handful of described factors in the 1970s, the cytokine catalog has expanded to more than 100 characterized members, with new families still emerging from functional proteomics and single-cell RNA sequencing screens.

What Are Cytokines? Classification and Core Families

Cytokines are low-molecular-weight signaling glycoproteins secreted by both immune and non-immune cells in response to a wide range of stimuli. They act locally (autocrine/paracrine) or systemically (endocrine) and exert pleiotropic, redundant, and sometimes antagonistic effects depending on the cellular context. The major cytokine families are:

Within the interleukin group, specific members have defined disease relevance: IL-1β activates the NLRP3 inflammasome; IL-2 drives T cell proliferation and Treg homeostasis; IL-4 and IL-13 together drive Th2 differentiation and IgE class switching; IL-6 orchestrates the hepatic acute-phase response and Th17 polarization; IL-10 is the canonical anti-inflammatory brake; IL-17A/F recruits neutrophils to mucosal sites; and the IL-12/IL-23 axis is the master switch for Th1 vs. Th17 polarization — a therapeutic target in psoriasis and Crohn's disease (ustekinumab, risankizumab).

Recently Discovered Cytokines: Expanding the Landscape

The boundaries of the cytokine family continue to shift. Several members characterized since 2005 have rapidly moved from discovery to clinical relevance:

IL-33 (2005, IL-1 family). Described by Schmitz et al. as a ligand for the ST2 receptor (IL1RL1), IL-33 is released as an alarmin by necrotic or mechanically stressed cells (Schmitz et al., Immunity 2005, PMID 16286016). It activates ILC2s, mast cells, and basophils — not T helper cells directly — making it a master driver of type-2 airway inflammation, atopic dermatitis, and allergic disease. The soluble form of its receptor, sST2 (shed extracellular domain of ST2/IL1RL1), acts as a decoy receptor that sequesters free IL-33 and is itself a circulating biomarker in heart failure and severe asthma.

IL-34 (2008). IL-34 was identified as a second, structurally unrelated ligand for CSF1R (CD115), the receptor shared with M-CSF. Although IL-34 and M-CSF bind to the same receptor, they differ in tissue distribution: IL-34 is the dominant CSF1R ligand in the CNS, where it drives microglial development and homeostasis independently of M-CSF. IL-34 is also expressed in skin keratinocytes, with roles in Langerhans cell maintenance. Its distinct expression pattern from M-CSF suggests tissue-specific roles in macrophage biology that remain under active investigation.

IL-36 family (IL-36α, IL-36β, IL-36γ, IL-36Ra). IL-1 family members that signal through the IL-36R/IL-1RAcP complex and are particularly active in epithelial tissues. IL-36γ is the dominant isoform in skin inflammation and is a key amplifier in pustular psoriasis (GPP), where spesolimab (anti-IL-36R) received FDA approval in 2022. The endogenous antagonist IL-36Ra mirrors the IL-1Ra/IL-1 structural relationship.

IL-37 and IL-38. Both are anti-inflammatory IL-1 family members. IL-37 signals via a complex of IL-18Rα and IL-37Rα (SIGIRR/TIR8) to broadly suppress innate immune activation. IL-38 is structurally similar to IL-36Ra and antagonizes IL-36 signaling; elevated in rheumatoid arthritis synovial fluid and implicated in IBD.

Meteorin-like protein (Metrnl; proposed IL-41). Metrnl is a secreted protein produced by adipose tissue and skeletal muscle. Some publications have proposed designating it IL-41, but this naming has not achieved consensus in IUIS/HGNC nomenclature as of this writing — "Metrnl" or "meteorin-like" remains the preferred designation. Functionally, Metrnl promotes type-2 immune responses through eosinophil-dependent IL-4 induction and exhibits anti-inflammatory effects in metabolic tissues.

Single-cell RNA sequencing has further accelerated cytokine discovery: computational tools (CellChat, NicheNet) systematically map cytokine–receptor pairs from scRNA-seq datasets, uncovering previously orphan ligand–receptor interactions and expanding the catalog of functionally characterized cytokines.

Cytokines in Disease: The Case of Cytokine Storm

A cytokine storm is a dysregulated, self-amplifying cascade in which feedback controls break down, leading to hyperinflammation and multi-organ damage. Contexts include:

• MAS/HLH: driven by IFN-γ and IL-18; ferritin >500 µg/L (MAS) to >10,000 µg/L (HLH).
• Severe COVID-19: elevated IL-6, CXCL10/IP-10, TNF-α, IL-18.
• CAR-T CRS: IL-6, IFN-γ, IL-1β; graded 1–4 by ASTCT 2018 criteria (Lee et al., Biol Blood Marrow Transplant 2019).

Therapeutic targets: tocilizumab and sarilumab block IL-6R (not IL-6 itself); anakinra is recombinant IL-1Ra blocking IL-1R1; canakinumab neutralizes IL-1β directly; emapalumab targets IFN-γ in HLH. Storm severity biomarkers (ferritin, IL-6, IL-18, CRP, IFN-γ) are quantifiable by ELISA. See Fajgenbaum & June, N Engl J Med 2020 for a comprehensive review.

Measuring Cytokines: ELISA, Multiplex Assays, and Beyond

Method choice depends on analyte count, sample volume, and sensitivity required:

• Single-plex sandwich ELISA — gold standard for individual cytokines; high specificity, quantitative, validated spike recovery in serum/plasma/supernatant. BioHippo distributes PicoKine® and EZ-Set™ kits with published sensitivity data.
• Multiplex bead-based immunoassays (Luminex Bio-Plex, MILLIPLEX, LEGENDplex; MSD electrochemiluminescence) — 10–80 analytes from 25–50 µL. MSD offers lower LOD than Luminex for low-abundance cytokines.
• Intracellular cytokine staining (ICS) by flow cytometry — single-cell resolution; qualitative to semi-quantitative.
• Single-molecule array (Simoa/Quanterix) — femtomolar sensitivity, 100–1,000× more sensitive than ELISA; used for neuroinflammation markers.
• ELISpot — single-cell cytokine secretion; standard for antigen-specific T cell responses and vaccine immunogenicity (IFN-γ IGRA for TB).

Pre-analytical variables: limit freeze-thaw cycles; use plasma (EDTA/heparin) over serum where possible; add protease inhibitors to lysates; discard haemolysed samples (haemolysis elevates IL-6 and IL-10).

Cytokine Research Tools at BioHippo

BioHippo distributes validated cytokine ELISA kits with sensitivity specifications and spike recovery data. Browse our full ELISA kit catalog or shop targeted kits:

• Human IL-6 ELISA Kit EZ-Set™ — 1–10 pg/mL sensitivity; validated in serum, plasma, and cell culture supernatant
• Human TNF-α ELISA Kit PicoKine® — <1 pg/mL sensitivity
• Human IFN-γ ELISA Kit EZ-Set™ — validated in serum and cell culture supernatant
• Human IL-1β ELISA Kit EZ-Set™ — 1–10 pg/mL; inflammasome activation assays
• Human IL-17A ELISA Kit PicoKine® — <1 pg/mL; Th17 and psoriasis research
• Human CXCL10/IP-10 ELISA Kit EZ-Set™ — validated in serum, plasma, saliva, and urine
• Mouse TNF-α ELISA Kit EZ-Set™ — murine sepsis and autoimmunity models
• Mouse IFN-γ ELISA Kit EZ-Set™ — tumor immunology and viral infection models

Also browse our High Sensitivity ELISA Kits (PicoKine® formats, <1 pg/mL LOD) and cytokine antibodies for functional studies.

Frequently Asked Questions

What are cytokines?

Cytokines are small secreted proteins (5–70 kDa) produced by immune and non-immune cells that act as intercellular messengers regulating inflammation, immune cell differentiation, hematopoiesis, and tissue homeostasis. They include interleukins, interferons, chemokines, tumor necrosis factors, colony-stimulating factors, and the TGF-β superfamily. Unlike classical hormones, cytokines are produced by many cell types and act primarily in a local (paracrine/autocrine) context, though systemic endocrine effects occur at high concentrations during severe infection or injury.

What is the difference between interleukins, interferons, and chemokines?

Interleukins (ILs) are a numerically named subgroup originally defined as signals between leukocytes; they encompass a functionally diverse set now numbered beyond IL-40. Interferons (IFNs) specialize in antiviral defense and immune modulation: Type I (IFN-α/β) signal via IFNAR1/2 to induce antiviral states; Type II (IFN-γ) activates macrophages and upregulates MHC II; Type III (IFN-λ1–4) act at epithelial barriers. Chemokines are ~50 small cytokines that direct leukocyte migration by binding GPCRs; classified by N-terminal cysteine arrangement (CC, CXC, CX3C, XC). All chemokines are cytokines, but not all cytokines are chemokines.

How are cytokines measured in research?

Single-plex sandwich ELISA is the most widely used method — quantitative, high specificity, ideal for 1–5 analytes. Luminex bead-based multiplex (10–80 analytes) and MSD electrochemiluminescence (lower LOD) enable panels from a single sample. Intracellular cytokine staining (ICS/flow cytometry) or ELISpot provide single-cell resolution. Simoa (Quanterix) achieves femtomolar sensitivity for ultra-low plasma concentrations. Key pre-analytical variables: freeze-thaw cycles, matrix (plasma vs. serum), haemolysis, and protease inhibitors.

What causes a cytokine storm?

A cytokine storm results when negative-feedback controls of immune activation fail, allowing IL-6, TNF-α, IFN-γ, and IL-1β to amplify in a self-reinforcing loop. Triggers include severe infection (sepsis, COVID-19), autoimmune flare (MAS/HLH), or iatrogenic immune activation (CAR-T CRS). The resulting hyperinflammation causes endothelial injury, capillary leak, coagulopathy, and multi-organ failure. Treatment targets the dominant mediating cytokine: IL-6R blockade (tocilizumab) for CAR-T CRS; IFN-γ neutralization (emapalumab) for HLH; IL-1Ra (anakinra) or anti-IL-1β (canakinumab) for MAS.

Why are cytokines important in drug development?

Cytokines are among the most validated therapeutic targets in medicine. Anti-TNF-α biologics (adalimumab, infliximab, etanercept) transformed the treatment of RA, psoriasis, and IBD. IL-6R antagonists (tocilizumab, sarilumab) are standard of care in giant cell arteritis and CRS. IL-17A/F inhibitors (secukinumab, ixekizumab) lead in ankylosing spondylitis and psoriatic arthritis. IL-23 inhibitors (risankizumab, guselkumab) dominate the psoriasis and Crohn's disease pipeline. Cytokine quantification by ELISA and multiplex assay underpins every stage of biologic drug development — from target validation and PD monitoring to patient stratification in clinical trials.